As is well known, the semiconductor technology has made remarkable advances toward higher integration of integrated circuits. This tendency promoted to use a light source of shorter wavelength in the lithography process for semiconductor device manufacture. Photolithography using ArF excimer laser (193 nm) is the current main stream. A transition to photolithography using extreme ultraviolet (EUV) is expected to enable further integration. As the technology for the fabrication of semiconductor devices with a half-pitch of 32 nm or less, not only the photolithography, but also the nanoimprint lithography are considered promising.
The nanoimprint lithography is expected to find a wide variety of applications including optical waveguides, bio-chips, and optical storage media.
The nanoimprint lithography involves furnishing a mold (also referred to as stamp or template) having a fine pattern predefined thereon by electron beam lithography and etching techniques, coating a resin material on a substrate, and forcing the mold against the resin film for transferring the configuration of the fine pattern to the resin film. Specifically, semiconductor devices are fabricated by forcing a mold against a resist film coated on the surface of semiconductor wafer such as silicon for transferring the fine pattern.
The nanoimprint lithography is generally divided into photo nanoimprint lithography and thermal nanoimprint lithography. The photo nanoimprint lithography uses a photo-curable resin as the resin material. While the mold is pressed against the resin, ultraviolet (UV) radiation is irradiated to the resin for curing, thereby transferring a fine pattern.
On the other hand, the thermal nanoimprint lithography uses a thermoplastic resin as the resin material. A fine pattern is transferred by pressing the mold against the thermoplastic resin which has been softened by heating above the glass transition temperature. Alternatively, a fine pattern is transferred by pressing the mold against a thermosetting resin while heating up to the curing temperature.
The properties required for nanoimprint molds include a mechanical strength to prevent failure of the mold during fine pattern transfer and a chemical stability to be inert to the resin.
The nanoimprint lithography is expected applicable to the fabrication of semiconductor devices with a half-pitch of 32 nm or less. However, the thermal nanoimprint lithography seems difficult to transfer a fine pattern at a high accuracy because the mold is heated by the same heat as applied for the softening or curing of the resin material so that the mold is deformed by thermal expansion.
It is thus believed that the photo nanoimprint lithography is selected when the nanoimprint lithography is applied to the fabrication of semiconductor devices with a half-pitch of 32 nm or less. In the photo nanoimprint lithography wherein UV radiation is transmitted by the mold, if the mold is UV absorptive, the mold temperature will vary. Also, the mold experiences temperature variations due to the heat of a light source or UV lamp, a temperature variation during nanoimprint process and other factors. In the transfer of fine patterns like semiconductor devices with a half-pitch of 32 nm or less, even a slight thermal expansion of the mold during the nanoimprint process can lead to a substantial decline of location accuracy. It would then be desirable to have a mold material that has a high transmittance and resistance to UV radiation and a low coefficient of thermal expansion.
JP-A 2006-306674 discloses to use as the mold material a low thermal expansion material having a high transmittance and resistance at the wavelength of a light source employed in the photo nanoimprint lithography.
The more precise transfer of a fine pattern, however, requires not only to use a low thermal expansion material having a high transmittance and resistance at the light source wavelength as the mold, but also to control the internal transmittance distribution of the mold at the light source wavelength. If an internal transmittance distribution exists within the mold, the resin can be cured to a varying extent upon light exposure, rendering the nanoimprint performance unstable.